docs: def _newton(self, orb, step):
          """Newton's Law of Universal Gravitation
          """
  
          date = orb.date + step
  
          new_body = zeros(6)
          new_body[:3] = orb[3:]
  
          for body in self.bodies:
              # retrieve the position of the body at the given date
              orb_body = body.propagate(date)
              orb_body.frame = orb.frame
  
              # Compute induced attraction to the object of interest
              diff = orb_body[:3] - orb[:3]
              norm = sqrt(sum(diff ** 2)) ** 3
              new_body[3:] += G * body.mass * diff / norm
  
          return new_body